In electric power steering systems, an assist torque substantially proportional to a steering torque is determined. The steering torque of a driver of an automobile is reduced by increasing a torque proportional gain that corresponds to the proportional relationship. At this time, if the torque proportional gain is too large, a control system oscillates to bring about steering wheel oscillations. In some cases, the degree to which the steering torque is reduced is therefore restricted. In order to solve this problem, an algorithm for suppressing oscillations by improving the phase characteristic of the control system through introduction of a phase compensator has been invented to prevent the steering wheel oscillations (refer to, for example, a reference literature 1).
For the purpose of improving oscillation suppression performance, in addition to the phase compensator, a control algorithm that suppresses oscillations by inferring an oscillation frequency component of a motor rotating speed from a steering torque signal and a current signal, which drives a motor, by means of an observer, and feeding back the oscillation frequency component has been invented to prevent steering wheel oscillations (refer to, for example, a reference literature 2).
Patent document 1: JP-A-8-91236 (p. 4, FIG. 1)
Patent document 2: JP-A-2000-168600 (p. 10, FIG. 12)